1. Field of the Invention
The present invention relates to an electron tube cathode, and more particularly to an electron tube cathode enhancing life characteristics and electron emissivity by forming one or more reducing metal layer(s) between electron emissive material layers.
2. Description of the Related Art
In general, an electron tube cathode is a part emitting thermions as an electron source in an electron gun of a cathode ray tube for TV or a camera tube. The electron tube cathode is manufactured by forming an electron emissive material layer composed of electron emissive materials on a metal base. An example of a conventional electron tube cathode is disclosed in Japanese Patent Publication No. Sho 64-5417. As shown in FIG. 1, this electron tube cathode has a cathode sleeve 1 in which a heater 3 is placed. A base 5 is mounted on the upper opening of the cathode sleeve 1, and an electron emissive material layer 7 is formed on the base 5. The cathode sleeve 1 is composed of nichrome material, while the base contains a reducing material such as silicon (Si) or magnesium (Mg) by 0.01 to 0.09 wt % and is composed of high purity nickel (Ni) as a main ingredient. The electron emissive material layer 7 is composed as main ingredients of alkaline earth metal oxides including at least barium (Ba) and in addition strontium (Sr) or calcium (Ca). The electron emissive material layer 7 also contains rare earth metal oxide such as scandium oxide by 0.1 to 20 wt %. The heater 3 emits thermions from the electron emissive material layer 7 by means of electrical heating.
In order to form the electron emissive material layer 7 in an electron tube cathode having the structure described above, barium carbonate (BaCO3), strontium carbonate (SrCO3), calcium carbonate (CaCO3) and a predetermined amount of scandium oxide (Sc2O3) are first mixed together with a binder and a solvent to prepare a suspension. The suspension is sprayed onto the base 5 to a thickness of about 800 xcexcm and thereafter heated by a heater during the cathode ray tube evacuating process. At this time, the carbonates of the alkaline earth metals are converted into alkaline earth metal oxides such as barium oxide (BaO), strontium oxide (SrO) and calcium oxide (CaO). Thereafter, a part of the alkaline earth metal oxides are reduced and activated so as to have semi-conductivity. Thus, the electron emissive material layer 7 composed of a mixture of the alkaline earth metal oxides and a rare earth metal oxide is formed on the base 5. In the activating process, the reducing materials such as silicon and magnesium, which are contained in the base 5, move to the interface between the alkaline earth metal oxides and the base 5 by diffusion, and react with the alkaline earth metal oxides. As a result of these reactions, a part of the alkaline earth metal oxides on the base 5 are reduced to be an oxygen deficient semiconductor, thereby facilitating electron emission.
In the electron tube cathode described above, a reducing material layer may be formed on the base by means of a sputtering method before forming a carbonate layer by means of a spraying method. In addition, in order to enhance life characteristics, a mixture layer of the carbonates and the reducing materials, which function to lower resistance of the non-conductive intermediate layer formed on the base, may be formed on the surface of the base, or a carbonate layer may be formed on the mixture layer.
However, the process of forming a reducing material layer on the surface of the base by means of the sputtering method poses a problem of failing to uniformly emit electrons, since the reaction between the reducing material and the base metal is not uniformly made over the entire base metal in the evacuating and activating processes. In addition, a difficulty is encountered in managing the manufacturing process, when forming a carbonate layer on the mixture layer of the reducing materials and the carbonates after forming the mixture layer. Moreover, forming the entire emissive material layer with a mixture of the carbonates and the reducing materials is likely to result in undesirable cathode condition patterns.
Japanese Patent Laid-Open No. Hei 2-75128 discloses a cathode in which an oxide layer of alkaline earth metal including barium is formed on a nickel base and contains scandium, and a metal layer containing at least one element selected from platinum (Pt), iridium (Ir) and rhodium (Rh) is formed between the nickel base and the oxide layer.
In the electron tube cathodes having the structure described above, although the rare earth metal oxides improves the supply of excess Ba, since the excess Ba supplying rate is controlled by the diffusion rate of the reducing material in the nickel base, the life characteristics of the cathode are greatly deteriorated in operating at a high current density over 2 A/cm2. Also, since the metal layer on the base is composed of a metal having a lower reducibility than tungsten (W) or molybdenum (Mo), it has almost no barium oxide reducing effect for enabling the operation at a high current density.
Japanese Patent Laid-Open Nos. Hei 3-230445 and Hei 2-267834 disclose a cathode composed of three emissive material layers. In Japanese Patent Laid-Open No. Hei 3-230445, a base is composed as a main ingredient of Ni containing reducing elements such as Si, Mg, etc., the first emissive material layer composed as main ingredients of alkaline earth metal oxides containing Sc2O3 of 0.05 to 5 wt % and Ba is formed on the base, the second emissive material layer composed as main ingredients of alkaline earth metal oxides including barium and containing at least one metal element selected from the 1B, 3B and 5B groups or oxide thereof by 0.01 to 5 wt % is formed on the first emissive material layer, and the third emissive material layer composed as main ingredients of alkaline earth metal oxides including barium is formed on the second emissive material layer.
In Japanese Patent Laid-Open No. Hei 2-267834, a base is composed as a main ingredient of Ni containing at least one reducing element, the first emissive material layer composed as main ingredients of alkaline earth metal including Ba is formed on the base, the second emissive material layer composed as main ingredients of at least one element selected from rare earth metal oxides, rare earth metals, heat resistant metal oxides and heat resistant metals is formed on the first emissive material layer, and the third emissive material layer composed as main ingredients of alkaline earth metal oxides including barium is formed on the second emissive material layer.
In case of a cathode having the structure consisting of three emissive material layers as described above, the reducing metal contained in the second layer exists in the form of a mixture with an emissive material. As mentioned above, however, this is likely to cause that the reaction between the reducing materials and the base metal may not uniformly occur over the entire base, and subsequently the electrons cannot be uniformly emitted. Also, the management of the manufacturing process is difficult, and undesirable cathode condition patterns are likely to be produced.
In Japanese Patent Laid-Open No. Hei 3-257735 and its counterpart U.S. Pat. No. 5,118,984, EP 445956, and Korean Patent Publication No. 93-11964, a base is composed as a main ingredient of Ni containing at least one reducing element selected from Si, Mg, W, zirconium (Zr) and aluminum (Al), a metal layer is formed on the base and contains at least one of W and Mo, and an emissive material layer composed as main ingredients of alkaline earth metal oxides including at least barium is formed on the metal layer and contains rare earth metal oxides of 0.01 to 25 wt %.
In this conventional cathode described above, as shown in FIG. 2, a heater 3 is arranged inside a cathode sleeve 1, a base 5 is arranged on the upper opening of the sleeve, an emissive material layer 7 is formed on the base 5, and a metal layer 9 is formed between the base 5 and the emissive material layer 7.
The cathode having the above-described structure produces a reactive product in addition to free barium atoms, and thus has stable characteristics at the beginning of use. As time elapse, however, the life characteristics are drastically deteriorated.
Korean Patent Laid-Open Nos. 99-58901, 99-58910 and 2000-20817 disclose a cathode comprising a metal layer formed between the base and an emissive material layer as a remedy to diffuse barium oxide, and a reactive product of silicon and Mg accumulated between the base and the emissive material layer, when generating a free barium atom.
In Korean Patent Laid-Open No. 99-58901, the base is composed of Ni as a main ingredient and contains at least one reducing element. A metal layer is formed on the base, and composed of W, Zrxe2x80x94W or Wxe2x80x94Ni. An emissive material layer is formed on the metal layer and contains alkaline earth metal oxides including at least barium. The emissive material layer may further contain lanthanum (La) compound and Mg compound at the same time, or may contain Laxe2x80x94Mg complex compound. Otherwise, an additional emissive material layer may be formed on the emissive material layer described above, which is composed of alkaline earth metal oxides including at least barium and contains La compound and Mg compound at the same time, or Laxe2x80x94Mg complex compound. Korean Patent Lai-Open No. 99-58910 discloses a cathode of a similar structure to that of Korean Patent Laid-Open No. 99-58901 except that the metal layer is composed of Ni as a main ingredient. Korean Patent Laid-Open No. 2000-20817 also discloses a cathode of a similar structure to that of Korean Patent Laid-Open No. 99-58901 except that the metal layer is composed of Ni, W, Nixe2x80x94Zr, Zrxe2x80x94W or Nixe2x80x94W as a main ingredient and has a concave portion in the upper surface for increasing the surface area. The cathode may further comprise an additional metal layer composed as a main ingredient of Ni, W, Nixe2x80x94Zr, Zrxe2x80x94W or Nixe2x80x94W on the base.
In Korean Patent Laid-Open No. 99-58901, 99-58910 and 2000-20817, however, a layer composed of W, Zrxe2x80x94W or Wxe2x80x94Zr is formed to have particles of smaller size than the average size of those in the base. Thus, despite a certain degree of diffusing effect of the intermediate layer, the cathode has a drawback of blocking a diffusing path of the reducing materials composed of Mg and Si as main ingredients due to formation of a barrier layer between the base and the emissive material layer.
In general, a cathode has a life cycle of about 10,000 hours. Most of Mg is consumed within the first 2,000 hours, and thereafter Si is mainly consumed as a reducing material. Of the numerous theories that have been suggested on the life cycle of a cathode, the most representative theory is that a moving amount of electrons is decreased due to a consumption of the reducing material as well as to an impediment of diffusion of the reducing material and an increase of resistance caused by an intermediate layer. As a result, there is a limit to enhance emission characteristics and life characteristics at a high current density.
It is, therefore, an object of the present invention to provide an electron tube cathode, which can enhance life characteristics at a high current density by forming a reducing metal layer between the emissive material layers.
It is another object of the present invention to provide an electron tube cathode, which can enhance emissive characteristics of electrons.
It is still another object of the present invention to provide an electron tube cathode, which can enhance thermal characteristics.
It is still another object of the present invention to provide an electron tube cathode, which can reduce loss of electric power.
To achieve the above object, there is provided an electron tube cathode according to an embodiment of the present invention, comprising: a base composed of Ni as a main ingredient and containing at least one reducing material; the first emissive material layer formed on the upper surface of the base and containing alkaline earth metal oxides including at least barium; a reducing metal layer formed on the first emissive material layer; and the second emissive material layer formed on the metal layer and containing alkaline earth metal oxides including at least barium.
Preferably, the metal layer may comprise at least one element selected from W, Mo, tantalum (Ta) and Ti.
The metal layer may further comprise any one of rhenium, yttrium or a mixture thereof. It is preferable that a total amount of any one of the above material is 3 to 5 wt % of the metal layer. It is also preferable that an amount of the metal layer is 8 to 15 wt % of the entire emissive layers comprising the first and second emissive material layers and the metal layer. The metal layer is preferably formed at the thickness of 3 to 5 xcexcm. The metal layer may be formed to be at least one or more layers by means of spraying method.
There is also provided an electron tube cathode according to another embodiment of the present invention, comprising: a base composed of Ni as a main ingredient and containing at least one reducing material; at least three or more emissive material layers formed on the base and containing alkaline earth metal oxides including at least barium; and at least two or more reducing metal layers formed between the emissive material layers, respectively.
Preferably, the metal layers may comprise at least one element selected from W, Mo, Ta and Ti.
Also, the metal layers may further comprise any one of rhenium, yttrium or a mixture thereof. It is preferable that a total amount of any one of the above material is 3 to 5 wt % of the entire metal layer. It is also preferable that a total amount of the metal layers is 8 to 15 wt % of the entire emissive layer comprising the emissive material layers and the metal layers. The entire metal layer is preferably formed at the thickness of 3 to 5 xcexcm. Each of the metal layers may be formed to be at least one or more layers by means of spraying method.
Therefore, according to the present invention, the reducing metal in the metal layer function to generate free Ba, thereby generating more amount of the free Ba in comparison with the conventional oxide cathode, and enhancing life characteristics and emissive characteristics at a high current density.
Further, use of a reducing metal in the metal layer also brings an effect of raising the temperature of the cathode due to an increase of the radiant heat of the metal layer within the emissive layer of the cathode comprising a metal layer and emissive material layers, thereby enabling an operation of the cathode at a lower operational temperature and reducing the electric power consumption while enhancing thermal characteristics.